Project Summary/Abstract Fractures are a major public health burden with its associated disability, cost, morbidity and mortality. In recent years, hip fracture rates are higher than expected and the incidence of vertebral fracture appears to be rising dramatically, especially after age 75 years. A growing body of research suggests that acidosis, even when subclinical, directly affects bone and can have detrimental effects on bone metabolism and health. Acidosis can inhibit osteoblast function and bone formation while promoting bone resorption and breakdown, thus impairing the bone?s ability to repair microdamage that occurs with daily wear and tear and accumulates with aging, and potentially contributing to higher fracture risk. The Nurses? Health Studies (NHS) I and II and the Health Professionals Follow-up Study (HPFS) are ongoing large-scale cohort studies with decades of follow-up and rich dietary and lifestyle data, and archived biosamples. Integrating metabolomics technology into population-based studies is emerging as a valuable research tool which could provide novel insights into cellular processes that affect fracture risk. Therefore, this proposal?s goal is to prospectively study the association between acid-base status, assessed through dietary acid load, plasma bicarbonate level and plasma metabolites, and risk of incident fracture. We hypothesize that perturbations in acid-base status through diet-dependent and independent mechanisms resulting in increased acidosis will be associated with higher fracture risk. We will prospectively examine the association of dietary acid load with risk of incident hip and vertebral fracture in NHS I and II and HPFS (Aim 1). We will use a nested case-control study of hip fracture cases (n=650) and matched controls (n=650) within these three cohorts to study the association between plasma bicarbonate level (Aim 2), plasma metabolites (Aim 3) and risk of incident hip fracture in women and men. Archived plasma samples collected pre-hip fracture diagnosis will be measured for metabolites using state-of-the art, high-throughput liquid or gas chromatography followed by mass spectrometry (LC/MS/MS and GC/MS) platforms. Using advanced computational and biostatistical methods in metabolomics and high-dimensional data analyses, we will use a targeted metabolomics approach as well as an agnostic approach to build distinct metabolite signatures using all of the available plasma metabolite data to distinguish hip fracture cases from controls. Ultimately, we expect these studies to produce new insights into the development of fractures that may lead to new approaches to their prevention and treatment.